High gloss (for example, >65%) attainment in paper coatings typically involves the use of plastic pigments which may be solid or hollow in form. The plastic pigments are “non film forming”, i.e., do not coalesce under ambient temperature and those encountered during the drying and finishing (calendaring or super calendaring) of coated paper. Hollow sphere plastic pigment (HSPP) has a core-shell morphology wherein the core is filled with water. During the paper coating drying process, the water in the void may diffuse through the shell and leave air voids. Due to the difference in refractive index between air and surrounding polymer shell, light is effectively scattered, contributing to coating opacity. The use of HSPP in paper coatings lead to improvements in gloss, brightness, and opacity and are considered to be more effective than solid plastic pigment (See, U.S. Pat. No. 6,410,158). In addition, print performance is often improved as a result of elevated sheet gloss, smoothness and opacity. The degree to which the sheet properties are improved is dependent on the particle size and void volume of the HSPP, other mineral pigments used, and the type and amount of finishing utilized. HSPP is used in the range of 2 to 20% by dry weight of pigment depending on the level of gloss desired. The remainder of the pigment portion of the coating is typically made up of inorganic materials such as kaolin, ground calcium carbonate (GCC), talc (minerals), titanium dioxide or precipitated calcium carbonate. The particle size of HSPP is typically in the range of 0.1 to 1.0 micrometers, and the particles are suspended in an aqueous phase and supplied at approximately 25-30% solids by weight.
Because HSPP is a significantly expensive pigment compared with typical mineral pigments used in paper coatings (at least 10 times more expensive than kaolin on a dry basis) and also more expensive than solid latexes (at least 25% more expensive on dry basis), paper manufacturers are always looking for more economically suitable alternatives. It would therefore be an advancement in the art to develop a suitable replacement for HSPP.